An extrusion such as extruded polyvinyl chloride (PVC) pipe is formed by feeding plastic into an extruder where it is subjected to high temperatures to create a molten substrate. The substrate then proceeds through a feed tube, at the end of which additional material known as capstock can be applied if desired. The process of adding additional material to the substrate is known as co-extrusion. The substrate and capstock are known as melt. The melt proceeds through an extrusion head, at the end of which the melt passes through a die. The die contains the circular cross-sectional profile shape to be extruded. The melt hardens as it exits the die in the desired cross-sectional form. The hardened material forms a tube that can grow to arbitrary length as additional melt is extruded.
Normally, PVC pipe is produced by extruding molten plastic through a single die of an extruder. In some cases, two PVC pipes are produced simultaneously by extruding molten plastic through a Y-block, a pair of extrusion heads and a pair of dies.
The ultimate shape of the extrusion is determined by a melt flow passage in the die between a bushing which surrounds a pin or mandrel. For circular pipe, the pin is circular in cross-section and an opening in the bushing which surrounds the pin is circular. To obtain a non-circular cross-section, such as a square, a circular extrusion is typically reshaped through a transition bushing and pin which are circular at the inlet and square, for example, at the outlet.
In accordance with the invention, there is provided a system and method for co-extruding two simultaneous melts. A co-extrusion system contains a primary extruder, a secondary extruder, a primary flow director, a secondary flow director, at least first and second co-extrusion assemblies and extrusion dies. The primary flow director divides the solid melt from the primary extruder into two symmetrical flow paths. The secondary flow director divides the melt from the secondary extruder into two symmetrical flow paths. The first and second co-extrusion assemblies co-extrude the melt simultaneously from the primary and secondary flow directors. The co-extrusion takes place ahead of the dies so that there is no substantial pressure change in the melt, thereby providing for consistent and uniform flow control. The dies receive the co-extruded melt from their respective co-extrusion assemblies.
The secondary flow director contains top and bottom plates, each of which have channels which form two symmetrical flow paths. Conduits extend from the bottom plate to feed the melt into the co-extrusion assemblies.